It is desirable for plastic films, particularly those used to store and transport materials, to be resistant to damage from sudden impact. Trash bags are a very good example where failure resistance to sudden impact is highly valued. Additionally, thin films that exhibit high strength requirements provide a better cost-performance relationship for the consumer. Currently, such films are most commonly produced from polyolefin films, including polyethylene films and preferably linear low density polyethylene (LLDPE) films.
For many years, high performance polyolefins, such as low density polyethylene (LDPE), have been readily available at a low manufacturing cost sufficient to justify commercial use in trash bags, including heavy duty garbage bags, leaf bags and trash can liners. The use of polyethylene, more particularly low density polyethylene, allows for the production of liners with remarkably thin gauge and flexibility while maintaining high strength characteristics such as puncture toughness and ultimate tensile strength.
More recently, linear low density polyethylene (LLDPE) has been used in place of conventional highly branched LDPE in many film applications, including bags or liners. LLDPE is widely recognized as being tougher and stronger than LDPE, thus contributing to reduced bag failures, including punctures and splitting under stress. In particular, LLDPEs made with metallocene or single site catalysts have been used to provide improved toughness.
Typically, dart impact resistance of an LLDPE based film at a constant film thickness is increased by using LLDPE's with either higher molecular weight or narrower molecular weight distribution (MWD) or lower density. Each of these options has some concerns. At a fixed extruder output, using a LLDPE with higher molecular weight or narrower MWD significantly increases the extrusion pressure, and the load on the extruder. If the film manufacturer is limited on extruder pressure, using higher molecular weight or narrower MWD LLDPE is not an option. On using an LLDPE with lower density, the stiffness of the film decreases and hence the film is perceived to be weak by consumers. It is of value to develop a film formulation that would run on existing extruders without causing any extrusion issues and not reduce film stiffness.
The present invention achieves this goal by providing a film formulation comprising a polymer and at least one organic filler, said polymer comprising a polyethylene polymer having a density less than 0.970 g/cm3 and a melting point less than 137° C. and said organic filler having elasticity lower than that of the polyethylene. The films can be characterized by having dart impact resistance at least 10% higher than that of the film without the organic filler.